On-line Solution Heat Treatment Process for Austenitic Stainless Steel Plates

ABSTRACT

An on-line solution heat treatment process for austenitic stainless steel plates is disclosed, which comprises continuous casting billet heating, high pressure water dephosphorization, rough rolling, finish rolling, softening process, ultra fast cooling, straightening, cutting to length, shot peening, pickling. The process of the present invention makes full use of the residual rolling temperature, without the need of reheating steel plates from the room temperature to the solid solution temperature. Compared with the conventional steel plate rolling process, the process needs to perform the softening process on the steel plates after rolling to ensure that the deformed austenite is fully softened before cooling. After the softening process, ultra fast cooling is performed to cool the steel plates from above 800° C. to below 430° C. so as to prevent intergranular chromium depletion caused by a large amount of precipitation of high chromium carbides from affecting the stain resistance of stainless steel.

TECHNICAL FIELD

The present invention belongs to the field of stainless steel heat treatment production processes, and particularly relates to a green on-line solution heat treatment process for austenitic stainless steel plates which makes full use of residual rolling temperature, saves energy and reduces emission.

BACKGROUND

China ranks first in the crude steel output in the world for a long term but still has a gap compared with the world advanced steel production countries mainly in aspects such as high carbon emission and energy consumption per ton of steel and unreasonable product structure. The steel industry of China has the CO₂ emission accounting for more than 15% of the total emissions of the whole country and thus will bear huge pressure to reduce emissions for a long time. Facing the increasingly drastic competition of the world steel industry, the steel industry of China must constantly adapt to green, intelligent and high-end demands and adjust the development direction of the industry.

Austenitic stainless steel is the most widely used type of stainless steel, has good heat resistance, stain resistance and thermal processing performance, and can be used in kitchen and bathroom supplies, auto parts, medical equipment, building materials, chemistry, food industry, agriculture, ship parts and other fields. Austenitic stainless steel plates are usually delivered in a solid solution state. At present, the solution heat treatment methods for austenitic stainless steel plates adopt an off-line solution heat treatment process, which requires to heat hot-rolled stainless steel plates from the room temperature to the solid solution temperature (usually 1050-1100° C.), preserve heat for a certain period of time to make carbides and various alloying elements fully and uniformly dissolved in austenite, then quickly conduct quenching to improve the stain resistance of stainless steel, soften the structure and eliminate work hardening, as shown in FIG. 1. The heating process of the off-line solution heat treatment process needs to consume a large amount of energy and emits harmful gases such as CO₂, which pollutes the environment.

SUMMARY

The present invention proposes a green on-line solution heat treatment process for austenitic stainless steel plates which makes full use of residual rolling temperature and has the characteristics of energy saving and emission reduction, reducing the production cost of austenitic stainless steel plates and the environmental pollution.

The main process flow of the on-line solution heat treatment process for austenitic stainless steel plates proposed by the present invention is shown in FIG. 2: continuous casting billet heating→high pressure water dephosphorization→rough rolling→finish rolling→softening process→ultra fast cooling→straightening→cutting to length→shot peening→pickling. It should be pointed out that the present invention is also applicable to a stainless steel medium and heavy plate hot-rolling production line with only one rolling mill, and is also within the protection scope of the patent.

The present invention adopts the following technical solutions:

An on-line solution heat treatment process for austenitic stainless steel plates, comprises the following steps:

Step 1, heating stainless steel continuous casting billets to 1200-1250° C. in a heating furnace, and preserving heat for 2-3 h to fully dissolve alloying elements and carbides;

Step 2, heating and preserving heat for the austenitized continuous casting billets, performing high pressure water dephosphorization after tapping to remove the surface iron scale, and then using a rolling mill to conduct multi-pass rolling to reach the target thickness required for a finished product;

Step 3, fully softening the steel plates which are work-hardened after rolling deformation to ensure that the product hardness does not exceed the standard requirements because hot-rolled stainless steel plates will be work-hardened; when heating and heat preservation conditions are available, heating the rolled steel plates to the solid solution temperature and preserving heat for 10-100 s; and when heating and heat preservation conditions are not available, to shorten the steel swinging time, improve the production efficiency and ensure the starting cooling temperature, selecting thin-gauge continuous casting billets with the thickness of ≤220 mm as raw materials in step 1, reducing rolling passes, ensuring that the finishing rolling temperature is above 1050° C., and swinging steel for 10-200 s after rolling;

Step 4, putting the stainless steel plates softened in step 3 in a cooling zone for ultra fast cooling to make the stainless steel plates quickly pass the temperature range of high chromium carbide precipitation; and inhibiting the precipitation of M₂₃C₆ to improve the stain resistance of stainless steel;

Step 5, putting the cooled steel plates with shape defect of strips such as warpage in a straightener for straightening;

Step 6, cutting the cooled and straightened steel plates to length according to the finished size requirements, and then transporting to a cooling bed for cooling;

Step 7, putting the steel plates cooled to the room temperature in a shot blasting machine for shot blasting, and then putting in a pickling tank for pickling to remove the surface iron scale and impurities.

Preferably, in step 3, when heating and heat preservation conditions are available, heating and heat preservation equipment is arranged between a finishing mill and a cooling zone or beside a steel rolling bay, and the heating and heat preservation method is one of induction heating, direct-fire heating or roller hearth type heating furnace.

Preferably, in step 3, when heating and heat preservation conditions are not available, steel is swung on the front mill table of the cooling zone after rolling; and to shorten the steel swinging time required for softening after rolling and improving the production efficiency, the deformation in the rough rolling process in step 2 accounts for more than 80% of the total deformation, and the deformation in the finish rolling process does not exceed 20% of the total deformation to reduce work hardening in the rolling process.

Preferably, in step 3, when heating and heat preservation conditions are not available, a heat preservation hood is arranged in the steel swinging zone to reduce temperature drop in the steel swinging process.

In step 4, for ultra fast cooling, the starting cooling temperature is not lower than 800° C., and the finishing cooling temperature is not higher than 430° C.

Preferably, in step 4, for ultra fast cooling, the starting cooling temperature is not lower than 850° C., and the finishing cooling temperature is 300-400° C.

Preferably, in step 4, the method for ultra fast cooling is high-pressure jet water cooling; and plates are cooled by passing the ultra fast cooling zone at one time or by swinging in the ultra fast cooling zone. In order to ensure the uniformity of cooling and the shape of plates after cooling, the mode of passing the ultra fast cooling zone at one time is preferred for cooling. For thick-gauge steel plates, if the one-time passing mode cannot meet the cooling temperature requirements, the swing mode can be selected.

The present invention has the following advantageous effects that: steel plates do not need reheating from the room temperature to the solid solution temperature, the deformed austenite structure is fully softened by making full use of the residual rolling temperature and then subjected to ultra fast cooling and quickly passes the high chromium carbide precipitation zone, and the precipitation of M₂₃C₆ is inhibited to improve the stain resistance of steel plates. The present invention realizes energy saving and emission reduction, and can shorten the manufacturing cycle of stainless steel solid solution plates and reduce the production cost. The on-line solution heat treatment process for stainless steel proposed by the present invention can be conveniently realized by partially modifying the existing medium and heavy plate production line.

DESCRIPTION OF DRAWINGS

FIG. 1 is a flow chart of an off-line solution heat treatment process for stainless steel;

FIG. 2 is a flow chart of the on-line solution heat treatment process of the present invention.

DETAILED DESCRIPTION

The following specific embodiment is intended to further describe the present invention. It should be pointed out that the protection scope of the present invention includes but is not limited to the specific embodiment listed below, and some non-essential modifications and adjustments made by others according to the present invention still belong to the protection scope of the present invention.

Embodiment

(1) 304 austenitic stainless steel continuous casting billets with the size of 180 mm×1536 mm (thickness×width) are heated to 1250° C. in a heating furnace and discharged after heat preservation for 2.5 h;

(2) The continuous casting billets discharged from the heating furnace are rolled after high pressure water dephosphorization, the initial rolling temperature is about 1220° C., the roughing mill reciprocates rolling for five passes, and the thickness of intermediate billets at the rough rolling exit is 26.0 mm; and the finishing mill conducts rolling for one pass to roll the intermediate billets to the end thickness, the finished size is 20.0 mm×1535 mm (thickness×width), and the finishing rolling temperature is about 1030° C.;

(3) The rolled steel plates are swung slowly on the front and back mill tables of the finishing mill for 150 s so as to fully soften the steel plates which are work-hardened after rolling deformation;

(4) The softened stainless steel plates are put in a cooling zone for ultra fast cooling, the starting cooling temperature is about 860° C., and the finishing cooling temperature is about 400° C.;

(5) The cooled steel plates are cut to length into a plurality of steel plates according to the finished size requirements, and the steel plates cooled to the room temperature on the cooling bed are transported to a shot blasting machine for shot blasting and then put in a pickling tank for pickling to remove surface impurities.

(6) The steel plates treated by the on-line solution heat treatment process is tested for performance, and compared with steel plates treated by the off-line solution heat treatment process for analysis. The grain sizes of the microstructures of steel plates treated by on-line and off-line solution heat treatment processes are respectively Grade 9.5 and 9.0, and the hardnesses are respectively 191 HB and 190 HB. The stain resistance of steel plates is tested according to Corrosion of Metals and Alloys-Test Methods for Intergranular Corrosion of Stainless Steels (GB/T4334-2008), the corroded and bent samples are observed under a 10× magnification microscope, and the surfaces of the samples are all intact without cracks. The performance test result shows that the on-line solution heat treatment process can obtain the product performance similar to that obtained by the off-line solution heat treatment process under the conditions of saving the production cost and shortening the production cycle. 

1. An on-line solution heat treatment process for austenitic stainless steel plates, comprising the following steps: step 1, heating stainless steel continuous casting billets to 1200-1250° C. in a heating furnace, and preserving heat for 2-3 h to fully dissolve alloying elements and carbides; step 2, heating and preserving heat for the austenitized continuous casting billets, performing high pressure water dephosphorization after tapping to remove the surface iron scale, and then using a rolling mill to conduct multi-pass rolling to reach the target thickness required for a finished product; step 3, fully softening the steel plates which are work-hardened after rolling deformation; when heating and heat preservation conditions are available, heating the rolled steel plates to the solid solution temperature and preserving heat for 10-100 s; and when heating and heat preservation conditions are not available, selecting thin-gauge continuous casting billets with the thickness of ≤220 mm as raw materials in step 1, reducing rolling passes, ensuring that the finishing rolling temperature is above 1050° C., and swinging steel for 10-200 s after rolling; step 4, putting the stainless steel plates softened in step 3 in a cooling zone for ultra fast cooling to make the stainless steel plates quickly pass the temperature range of high chromium carbide precipitation; step 5, putting the cooled steel plates with shape defect of strips such as warpage in a straightener for straightening; step 6, cutting the cooled and straightened steel plates to length according to the finished size requirements, and then transporting to a cooling bed for cooling; step 7, putting the steel plates cooled to the room temperature in a shot blasting machine for shot blasting, and then putting in a pickling tank for pickling to remove the surface iron scale and impurities.
 2. The on-line solution heat treatment process for austenitic stainless steel according to claim 1, wherein in step 3, when heating and heat preservation conditions are available, heating and heat preservation equipment is arranged between a finishing mill and a cooling zone or beside a steel rolling bay, and the heating and heat preservation method is one of induction heating, direct-fire heating or roller hearth type heating furnace.
 3. The on-line solution heat treatment process for austenitic stainless steel according to claim 1, wherein in step 3, when heating and heat preservation conditions are not available, steel is swung on the front mill table of the cooling zone after rolling; and to shorten the steel swinging time required for softening after rolling and improving the production efficiency, the deformation in the rough rolling process in step 2 accounts for more than 80% of the total deformation, and the deformation in the finish rolling process does not exceed 20% of the total deformation to reduce work hardening in the rolling process.
 4. The on-line solution heat treatment process for austenitic stainless steel according to claim 1, wherein in step 3, when heating and heat preservation conditions are not available, a heat preservation hood is arranged in the steel swinging zone to reduce temperature drop in the steel swinging process.
 5. The on-line solution heat treatment process for austenitic stainless steel according to claim 1, wherein in step 4, for ultra fast cooling, the starting cooling temperature is not lower than 800° C., and the finishing cooling temperature is not higher than 430° C.
 6. The on-line solution heat treatment process for austenitic stainless steel according to claim 1, wherein in step 4, the method for ultra fast cooling is high-pressure jet water cooling; and plates are cooled by passing the ultra fast cooling zone at one time or by swinging in the ultra fast cooling zone. 